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Understanding the term “SMART GRID”

16 Jan

The grid is a network that connects electric power generation plants to high voltage lines which carry electricity over some distance to substations. When a destination is reached, the substations decrease the voltage for “distribution” to medium voltage lines and then further to low voltage lines. Finally, a transformer on a telephone pole reduces it to a household voltage of 120 volts. See diagram below. The overall grid can be thought of as composed of three major sections: generation (plants and step up transformers), transmission (lines and transformers operating above 100,000 volts – 100kv) and distribution (lines and transformers under 100kv). Transmission lines operate at extremely high voltages 138,000 volts (138kv) to 765,000 volts (765kv). Transmission lines can be very long – across state lines and even country lines. For the longer lines, more efficient high voltages are used. For example, if the voltage is doubled, the current is cut in half for the same amount of power being transmitted. Line transmission losses are proportional to the square of the current, so long line losses are cut by a factor of four if the voltage is doubled. “Distribution” lines are localized across cities and surrounding areas and fan out in a radial tree-like fashion. This tree-like structure grows outward from a substation, but for reliability purposes, it usually contains at least one unused backup connection to a nearby substation. This connection can be quickly enabled in case of an emergency so that a substation’s territory can be fed by an alternative substation.

 

For back up purposes, transmission lines run in parallel and are interconnected with other transmission systems. In the US there are three major interconnected grid systems: those east of the Rocky Mountains, those west of the rocky mountains, and the state of Texas. However, back up is a two way street. A catastrophic failure in one part of the grid can bring down a substantial section of the interconnected grid as happened in 2003 when a tree in Ohio brought down the grid all the way to New York City and parts of Canada. Since then operational improvements have been incorporated that hopefully will prevent another large domino-effect.

In the grid system, electricity is used in real time as it is generated. That is, energy is being generated and used simultaneously in the same amounts. Supply and demand on the grid must always be in balance. There is almost no ability to store electricity. Also it is difficult for the grid to accommodate large, rapid changes in either load or generation. Obviously the load fluctuates quite a bit – both daily and seasonally. However, with many cities connected together, small fluctuations tend to cancel each other out. The aggregate demand normally behaves in a relatively predictable manner. Based on the day of the year from historical data and current weather forecasts, a next-day “load forecast” is usually made by noon of the previous day. A minute by minute schedule is put together for the generating plant and is adjusted in real time as the day progresses. Very skilled operators monitor the actual load and make minute by minute adjustments. They also watch carefully for any unusual happenings and are trained to act accordingly.

Grid critics have long claimed that the nation’s aging electrical network can not support the enormous expansion of wind and solar power coming on stream as renewable resources are being dictated by many states and supported by the US Govern-ment. Solar and wind are somewhat unpredictable, especially wind. In Wyoming for example, huge wind plants are coming on board in the next few years. This will cause large errors in the “daily forecasts” and make it difficult to manage the supply to the load if the wind suddenly slows. If the wind suddenly dies, the load has to be taken up by standby gas plants. Because these traditional plants take considerable time to power up, they must be run in “spinning standby”, i.e running (and polluting) but not putting out any electricity. The heat from spinning standby is dumped into their cooling towers instead of generating electricity. These are some of the challenges facing the smart grid.

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Posted by on January 16, 2012 in RENEWABLE ENERGY

 

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